Free Flyback Converter Calculator

Flyback Converter Design: Duty Cycle, Peak Current, and Inductance

The Flyback Converter Calculator is an online tool designed to help power electronics engineers quickly determine the duty cycle, peak current, and inductance values for both the primary and secondary sides of a flyback transformer. Whether you are stepping up or stepping down the voltage, this DC-DC converter calculator handles the iterative computations and gives you reliable design parameters in seconds.

What Is a Flyback Converter?

A flyback converter is a buck-boost topology that provides galvanic isolation between the input and output circuits. It uses a coupled inductor (often called a flyback transformer) with a gapped core to store energy while the switching element (typically a MOSFET) is on, and then releases that energy to the output during the off period. This isolated DC-DC conversion makes the flyback converter a popular choice for chargers, adapters, auxiliary power supplies, and other low‑power applications.

How Does the Flyback Transformer Work?

The key component is the flyback transformer, which behaves as a coupled inductor. During the on‑time ($t_{\mathrm{on}}$), the primary winding builds up current and stores magnetic energy in the core. The secondary diode is reverse‑biased, so no current flows to the load. When the MOSFET turns off, the magnetic field collapses, inducing a voltage across the secondary winding. The diode now becomes forward‑biased and the stored energy is transferred to the output capacitor and load. The cycle repeats at the switching frequency $f_s$.

Key Electrical Parameters

Duty Cycle

The duty cycle $D$ is the fraction of one switching period $T = 1/f_s$ during which the switch is on:

$$D = \frac{t_{\mathrm{on}}}{T} = t_{\mathrm{on}} \cdot f_s$$

Alternatively, $D$ can be expressed directly from the circuit voltages and the turns ratio $N = N_p / N_s$:

$$D = \frac{(V_{\mathrm{out}} + V_{\mathrm{rect}}) \cdot N}{V_{\mathrm{in}} + (V_{\mathrm{out}} + V_{\mathrm{rect}}) \cdot N}$$

where:

• $V_{\mathrm{in}}$ = input voltage,
• $V_{\mathrm{out}}$ = desired output voltage,
• $V_{\mathrm{rect}}$ = forward voltage drop of the output diode,
• $N$ = primary‑to‑secondary turns ratio.

Peak Current and Inductance – Secondary Side

For the secondary winding, the peak current $I_{\mathrm{s,peak}}$ and the required inductance $L_s$ are linked by the output current $I_{\mathrm{out}}$ and the off‑time:

$$I_{\mathrm{s,peak}} = \frac{2\,I_{\mathrm{out}}}{1 - D}, \qquad L_s = \frac{(V_{\mathrm{out}} + V_{\mathrm{rect}})(1 - D)}{I_{\mathrm{s,peak}} \, f_s}$$

Peak Current and Inductance – Primary Side

On the primary side, the peak current $I_{\mathrm{p,peak}}$ and the primary inductance $L_p$ depend on the input voltage, duty cycle, and switching frequency:

$$I_{\mathrm{p,peak}} = \frac{V_{\mathrm{in}}\, D}{L_p \, f_s}, \qquad L_p = \frac{V_{\mathrm{in}}\, D}{I_{\mathrm{p,peak}} \, f_s}$$

These equations form the foundation of flyback converter design. Once you have the duty cycle and the currents, the calculator can also output the core’s required inductance values for both windings.

Practical Design Example

To see the tool in action, take the following design specifications:

Parameter	Value
Input voltage $V_{\mathrm{in}}$	95 V
Output voltage $V_{\mathrm{out}}$	12 V
Diode forward drop $V_{\mathrm{rect}}$	1 V
Turns ratio $N$	5.3
Output current $I_{\mathrm{out}}$	4 A
Switching frequency $f_s$	70 kHz

Entering these values into the Flyback Converter Calculator yields:

• Duty cycle $D$ = 42.04 %
• Primary peak current $I_{\mathrm{p,peak}}$ = 2.60 A
• Secondary peak current $I_{\mathrm{s,peak}}$ = 13.80 A
• Primary inductance $L_p$ = 219.08 μH
• Secondary inductance $L_s$ = 7.80 μH

These results show how the tool simplifies the otherwise tedious algebra of flyback design.

Using the Calculator Forward and Backward

The calculator is not limited to computing $D$, $I_{\mathrm{peak}}$, and $L$ from a given set of input/output specs. It can work backward as well: if you already have a target peak current or inductance, you can supply those values and find the required switching frequency, turns ratio, or voltage levels. This bidirectional functionality makes it a versatile aid for iterating through different design constraints.

Whether you are a novice learning DC-DC flyback converter concepts or an experienced engineer finalizing a power supply, this flyback transformer calculator provides immediate numbers for the most critical parameters: duty cycle, peak current, and inductance on both sides of the isolation barrier.